In the manufacture of steel products such as steel plates, steel shapes and rails, some periodic surface irregularities occur in the material and sometimes such irregularities are so great that the material is not according to the specifications. Such unsatisfactory flatness must be accurately measured to adjust the settings of the rolling mills, etc. on the manufacturing line. Particularly, where the period of irregularities is relatively short, there is a great necessity for straightening the irregularities and the irregularities must be measured with a high degree of accuracy.
The flatness measuring methods which have heretofore been used for this purpose include for example a method that employs an electrical level and another method that employs a differential transformer displacement measuring device so that the flatness of a material to be measured which is placed on a surface plate is measured in terms of variations in the height of the material. However, these conventional methods have their own disadvantages as will be described hereunder.
In other words, the measuring method employing an electrical level is disadvantageous in that since the flatness of a material to be measured is obtained by measuring the degree of levelness of the measured surface at regular intervals of about 200 mm by applying the electrical level to the surface of the material, not only is the measuring operation complicated and time consuming, but also the measurement is affected by the conditions in which the material is placed or the degree of levelness of its holding stand and the greater curvature of the material on the whole (hereinafter referred to as an overall curve), thus making it impossible to make an accurate measurement of the actual irregularities in the surface of the material. The measuring method of measuring the flatness of a material by a differential transformer displacement measuring device in terms of variations in the height of the measured surface is also disadvantageous in that since the measurement is carried out by placing a material to be measured on a surface plate having a very high degree of flatness and moving the displacement measuring device in a direction parallel to the surface of the surface plate, while it is necessary to provide a surface plate device having an excellent flatness, it is difficult to ensure the required accuracy of a surface plate or obtain a suitable surface plate that meets a long measuring length, thus giving rise to a difficulty in providing the necessary equipment. Moreover, the fact that the location of measurement is limited to the location of the surface plate gives rises to difficult problems in operation such as the transfer of material to be measured. There is also a deficiency in the efficiency of the method in that even if the material is measured by placing it on a surface plate the measurement is affected by the overall curve of the material.